This invention relates generally to generators for producing smoke to disperse fungicidal, insecticidal, bactericidal and other disinfecting and sterilizing agents, and more particularly to a sealed container having both a smoke-generating pesticidal mixture and an internal heater therein for activating the mixture.
The term pesticide is now officially used to cover all toxic chemicals capable of destroying or inhibiting the activity of plant or animal pests. This term therefore encompasses insecticides, herbicides, fungicides, rodenticides and other disinfecting or sterilizing agents.
It is well known that the most effective treatment for predators encountered in the manufacturing and storage facilities of the food industry as well as in the transportation of perishables, involves the dispersion into the atmosphere of vapors containing minute particles of active pesticidal agents. Due to their very light weight, these particles float in the air and make contact with the contaminants being treated.
Commonly used for this purpose is a smoke-producing mixture having a base of ammonium nitrate and dicyandiamide. Once the mixture is heated to the proper temperature, an exothermic reaction occurs, which action is then independent of the heat source. It is therefore necessary to provide a heat source capable of converting the mixture into smoke. This conversion poses problems which heretofore have not been solved in a satisfactory manner.
Among the various techniques hitherto employed for heating the smoke-generating mixture are electric and open-flame or gas heaters. It is also known, as disclosed in French Pat. No. 1,400,481, to provide a heater in the form of an auto-combustible material molded into a receptacle for containing the gas-generating mixture. The drawback to this approach is that during prolonged storage, the mixture diffuses into the auto-combustible material of the receptacle. To overcome this drawback, French Pat. No. 1,439,921 discloses an arrangement in which the smoke-generating mixture is contained within a thin-walled metal can, the can being nested within the receptacle formed of auto-combustible material.
In the case of an electric heater or of a auto-combustible receptacle adapted to heat a metal can having a smoke-generating mixture therein, the heat source is exterior to the can and the heat must be transmitted from the exterior to the interior of the can. As a consequence, a substantial portion of the available heat is dissipated into the ambient atmosphere by radiation and convection, the remaining portion being conducted to the active mixture within the can container.
Moreover, with a receptacle made of auto-combustible material surrounding the metal can, despite the fact that the mass of this material is very large (i.e. -- 270 grams), the resultant heating is irregular or erratic. If the smoke-generating mixture is heated too abruptly, the active ingredients therein deteriorate and the insecticidal, bactericidal or fungicidal effectiveness thereof is markedly diminished, whereas if the mixture is insufficiently heated, the emission of smoke is incomplete and a significant quantity of unvolatilized material remains on the bottom of the metal can.
A further objection to the use of receptacles made of auto-combustible material for heating the metal can is that the heating efficiency will vary according to the prevailing climate, the season of the year and the ventilating conditions of the chamber being fumigated. For example the same receptacle which works in a perfectly satisfactory manner a food warehouse situated in the south of France, will perform poorly in a warehouse belonging to the same company but located in the north of France. These irregularities are even more marked in well-ventilated silos situated in North Africa or in the Scandinavian countries.
Still another objection to receptacles formed of combustible material is that it constitutes a fire hazard and its use may require special safety precautions. This factor has militated against the widespread use of such combustible receptacles for heating fumigants.